Mailing list managers should be more or less irrelevant. AFAIK they all store any archived emails just like a client email reader does, in one of three or four different standard file formats. At least for the first pass that's all that matters. As the project actually gets going we'd want to look into that more, as it's likely that different mailing list manager programs insert additional headers into the emails which may contain useful metadata.
I have no opinion yet whether this new functionality should be added into the existing Forums package, constructed as a second package, or what. I definitely think that the existing Forums data model, code, and especially UI should be re-used as much as is possible and practical. Web-based forums, email lists, and Usenet groups are all merely differently flavored implementations of the same basic concept, the "discussion list", and the software design should reflect that. I seek to unify them.